Putting that aside along with the usual dirt and grass that's right outside the pool that everybody walks on, you usually have a number of people that do serious swimming in the pool. Regardless of the fact that they're surrounded by water while they swim, their bodies will still overheat and start to produce sweat (especially if they get out of the pool to rest). I really don't see a need to go into the pool any cleaner than you would if you were going to work or school. Compared to the grass and dirt that usually treads its way in the pool, the occasional urine from children, and the sweat from active swimmers, the amount of grime that the common person would drag in avoiding a pre-swimming rinse off is minimal.

Am I going to break out in a huge rash because my body doesn't know how to soak in Japanese water?

actually-- this brings up a good question, at least for me. I've had softened and filtered water service for years in my house, and when I go on vacation the water often does cause me troubles. This is especially more noticeable in rural areas, grandma's house is bad, but Santa Monica wasn't so bad. It's nowhere near "breaking out in a huge rash" bad, but how is the water in Japan (especially for anyone who knows what I'm talking about here)? Also, I mentioned, the water is also filtered. I can't stand to drink tap water anymore, only filtered water and a few brands of bottled water. How's the taste of tap water in Japan, and are filter services, or at least those things you attach on to the end of your faucet, easy to find in Japan?

as to the thread itself, I know this is one bit of Japanese culture that will forever remain locked out to me. I'm a very private person, I lock my door before I change clothes, even if I'm the only one home. I've never even let a doctor see me naked (at least, since I was around 5..). I'm the kid that changed in shifts in gym, so that I'd always have at least a shirt covering myself up if I didn't have pants on.